I've also got a Mignon and it honestly does get easier/faster to dial in as you get to know you setup... so it really is just building up experience. A couple of things that I've learned. I try and keep it simple and consistent. 1:2 ratio - 18g into the basket and 36g into the cup shooting for extraction time between 25 and 30s. With the correct grind size setting my grind timer is generally between ~10s and ~11s to get the 18g into the portafilter.
Once you are in the ball park, it becomes easier to dial in from roast to roast. When I'm making changes on the same bean / similar roast I literally move the dial by the smallest amount possible. Going from a medium roast to a light roast I'll have to change the dial between a half number to a whole number on the grind size dial... doesn't need more than that. Also, when adjusting the grind finer, do so while the grind is running - that avoids the burs from seizing up and the motor stalling out because you've clamped down too firmly on some coffee already in the burrs and it helps purge some of the older coarser coffee that may screw with consistency.
I've also found that if I don't want to fiddle with the grind settings anymore adding or subtracting a couple of decimal points from the grind timer is also a great way to fine tune that final bit. e.g. shot is extracting good time wise (so I don't want to grind any coarser/finer), but I may want to dial out a bit of sourness I can dose slightly less by taking 0.3 seconds out of the grind time to try and improve extraction to see if that makes a difference.
I'm still learning as well, so these are just my experiences and definitely not meant as gospel. So take them with a grain of salt if you will.
Finally, I've got one of these scales . It's cheap, accurate and small enough that it fits in easily on drip tray with a cup on top.
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